Chemical composition: Hornblende is a member of the amphibole family. Its atomic structure is similar to other amphiboles, except that there is a large vacant site, which is occupied by K or Na. Hornblende is a silicate of aluminium, calcium, magnesium and iron with sodium.
The composition is variable and represented by the formula (K, Na)0| (Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8022(0H)2. Many end members have names as given in Table 4.17.
Physical properties:
(i) Form: Massive, bladed, columnar, and fibrous. Crystallised varieties are prismatic.
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(ii) Colour: Black, dark green or greenish black.
(iii) Lustre: Vitreous, fibrous varieties are silky.
(iv) Streak: White.
(v) Hardness: 5-6.
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(vi) Cleavage: Perfect two sets.
(vii) Fracture: Uneven.
(viii) Specific gravity: 2.7 – 2.9.
(ix) Diaphaneity: Transparent to translucent.
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Use: Building stone, mosaic flooring. 4.3.22. Augite
Chemical composition:
Agate is a member of the pyroxene family. Its chemical structure is similar to other pyroxenes, complex and variable and represented by the formula (Ca, Mg, Fe, Na,) (Mg, Fe, Al) (Si, Al) 206. Ti, Li, Mn and other elements may also be present in the crystal lattice in small amounts.